I bought one of the charred five gallon rum barrels sold here on the forum. After a couple of beers being aged in there (refilling with a cup of rum when stored between batches), I would prefer to change it over to whiskey. Each time I put a cup of rum in there the barrel would absorb it over a month or so. That is why I thought it might work. Any thoughts?

Bourbon and whiskey almost always uses a new barrel (always with bourbon), whereas scotch, tequila, and rum will use once used bourbon barrels. Id say no or you will almost certainly pick up off flavors.